My betta, Ariel, has this ??? i dont even know.. but this tiny part of the back of his tail is gone.. i 1st noticed it when he was upset about us getting him a snail. i quickly dismissed it as a temper tantrum. The snail has been gone for a while and the little part may possibly getting worse (slowly)
since the picture isnt the greatest quality, i'll describe it then post a pic
Ariel is a vibrant red with very flowy fins. The part in question is void and the edges of the void is a darker red possibly even black. The edges of the rest of his fins are translucent. I sure hope it was simply a temper tantrum and not some sort of disease.

Do you have any decorations with sharp edges? Each decoration should pass a panty hose test for those fish with long fancy flowy fins! Could also be fin biting, fin rot, etc. That picture is reallllly hard to tell anything from though!

im 99% sure this was the result of tail biting, probably because of the snail. he may do it again or he may not, i had one fish for over a year and then last month he started eating his tail for no reason. just keep his water super clean and he will heal up in no time!